Metallic surfaces which are constantly immersed in water can be given very effective protection against corrosion by simple and relatively cheap means. With the use of cathodic protection in the form of sacrificial anodes or in the form of impressed current, large steel structures can be kept free from corrosion. For this reason, drilling platforms and production platforms are normally not painted on the areas which are submerged in water. Ships with effective cathodic protection can be completely free from corrosion even on large areas of a flat bottom where the paint has been removed after touching bottom.
These examples show how effective cathodic protection of metal can be under water. Also, buried pipe lines and tanks, both offshore and onshore, can be protected by the use of cathodic protection, even though in this case normally combined with coatings of different types.
Tanks, containers, and pipe lines used with circulating water, for instance, can be protected inside by the addition of inhibitors, either inorganic or organic. The term "inhibitors" includes substances which remove oxygen from the water, such as sodium sulfite and hydrazine. Corrosion protection can also be obtained by making the water alkaline.
On surfaces in open air, particularly in marine and industrial environments, other normally more expensive methods must be used. On steel, the most usual method is galvanizing or painting, or eventually a combination of the two. In these cases the steel must normally be either pickled or sandblasted prior to the application of the corrosion preventing coating.
However, corrosion problems are particularly serious on surfaces in marine or industrial environments which are alternately dry and wet. Examples are the splash zones on structures in the sea, pipes, and tunnels, and the like which are particularly exposed to condensation. On such surfaces paints, for example, perform very poorly.
Asbestos cement pipes lead to a different type of problem, such as danger to people where the water supply can pick up asbestos pollution. The coating of asbestos cement pipes has heretofore been difficult; for many coatings are themselves dangerous and most of them require drying the pipes, for wet surfaces are difficult to coat effectively.